Gas turbine engines including a can-annular combustion system comprise a compressor and a turbine. The can-annular combustion system comprises a plurality of combustor assemblies and a like number of transition ducts. In one design, these combustor assemblies comprise a combustor casing, a burner assembly, and a combustor liner. Each transition duct connects a corresponding combustor liner to an inlet of the turbine. Compressed air enters each combustor assembly from the compressor, and is mixed with fuel in the burner assembly. The fuel and air mixtures burns within the combustor liner and transition duct, and the combustion products exit the transition duct into the turbine. The coupling of heat release oscillations with the acoustics of the combustor assembly is known to cause combustor acoustic pressure oscillations. These pressure oscillations can occur over a wide range of frequencies, depending upon the geometry of the combustor assembly and the heat release profile within the combustor assembly. These pressure oscillations in the combustor assembly can cause high cycle fatigue, leading to reduced life of combustion assembly components or restricted engine operation.
One known method for controlling combustion acoustic pressure oscillations is to incorporate Helmholtz resonator assemblies into the liner. These resonator assemblies are commonly used to damp high frequency pressure oscillations in gas turbine combustor assemblies. Because the resonator assemblies for controlling high frequency pressure oscillations are typically compact, they can be easily located on the combustor assembly liners. A known resonator assembly comprises a resonator outer plate having a plurality of generally circular openings closely spaced relative to one another and positioned over substantially the entire surface area of the outer plate, a resonator side wall coupled to the resonator outer plate, and a resonator inner plate defined by a portion of the liner. The resonator inner plate is provided with a plurality of closely spaced openings that are located over substantially the entire surface area of the inner plate. Air is supplied through the openings in the outer plate, into an inner cavity defined by the resonator inner and outer plates and side wall and then through the openings in the resonator inner plate. The plurality of resonator assemblies are spaced apart circumferentially about the liner and are generally positioned in alignment in an axial direction.
A thermal barrier coating is applied to a substantial portion of the inner surface of the liner to protect the liner from the hot combustion products passing therethrough. However, the thermal barrier coating can lengthen a neck of each Helmholtz resonator assembly, thus altering its damping performance. Therefore, prior to applying the thermal barrier coating to the liner inner surface, masking material is typically applied over the area where the openings are located so as to prevent thermal barrier coating material from being applied to the inner surfaces of the resonator inner plates. Since the resonator inner plates include a plurality of closely spaced openings, it is impractical to mask only the areas adjacent to the openings while leaving the areas between the openings unmasked. Therefore masking material is typically applied in a circumferential band to the inner surface of the liner. This masking technique prevents thermal barrier coating material from being applied in the areas adjacent to the resonator assembly openings, but also prevents the thermal barrier coating from being applied to the areas between resonator assemblies. Those unprotected portions of the liner inner surface are exposed to the hot combustion products passing through the liner and, as a result, require cooling air that flows through the resonator assemblies. A minimum amount of cooling air is required to prevent overheating of the liner, which may result in thermal fatigue of the liner and part failure.